The present disclosure concerns a geared gas turbine engine and in particular to a geared turbofan gas turbine engine or a geared turbo propeller gas turbine engine.
A geared turbofan gas turbine engine or a geared turbo propeller gas turbine engine comprises a gearbox which is arranged to drive the fan or propeller. The gearbox allows the fan, or the propeller, to rotate at a speed less than the speed of rotation of a turbine driving the gearbox. This enables the efficiency of the fan, or the propeller, and the efficiency of the turbine to be improved.
In one arrangement the gearbox comprises a sun gear which is arranged to be driven by a turbine, an annulus gear which is arranged to be static, planet gears meshing with the sun gear and the annulus gear and a carrier which is arranged to drive the fan, or the propeller, if the gearbox is a planetary gearbox. In another arrangement the gearbox comprises a sun gear which is arranged to be driven by a turbine, an annulus gear which is arranged to drive the fan, or the propeller, planet gears meshing with the sun gear and the annulus gear and a carrier which is arranged to be static if the gearbox is a star gearbox. In a further arrangement the gearbox comprises a sun gear which is arranged to be driven by a turbine, an annulus gear which is arranged to drive a first fan, or a first propeller, planet gears meshing with the sun gear and the annulus gear and a carrier which is arranged to drive a second fan, or a second propeller, if the gearbox is a differential gearbox. In another arrangement the gearbox comprises a sun gear which is arranged to be driven by a turbine, an annulus gear which is arranged to drive a fan, or a propeller, planet gears meshing with the sun gear and the annulus gear and a carrier which is arranged to drive a compressor, if the gearbox is a differential gearbox.
In large, high speed, epicyclic, or planetary, gearboxes the gearbox must endure very high loads due to centrifugal loading from the rotating planet gears and the basic torque load which the gearbox is arranged to transmit. The carrier of the gearbox is required to support the loads applied to the planet gears and planet gear bearings, which may be generated by torque or centrifugally generated. The carrier must also maintain the positions of the gears very accurately to maintain adequate gear performance in terms of controlling the tooth loading and the noise, or vibration, levels.
In large, high speed, epicyclic, or planetary, gearboxes the centrifugal load of the planet gears on the planet gear bearings is very high making it difficult, or impossible, to use rolling element bearings. It has been proposed to increase the diameter of the planet gear bearings to carry the centrifugal load of the planet gears but this increases the diameter and mass of the planet gears. However, the increase in bearing capacity due to the increase of bearing diameter is offset by the increase of mass and centrifugal loads of the planet gears. It has also been proposed to use journal bearings, as an alternative to rolling element bearings, for the planet gears, but journal bearings are susceptible to damage if there is an interruption in lubricant supply.
Furthermore, the planet gears may suffer from stresses due to ring bending in the gear teeth roots. Ring bending may cause the planet gears to become non-circular and this adversely affects the circumferential load distribution in the bearing and introduces bending fatigue. In the case of rolling element bearings ring bending adversely affects the cage to bearing race contacts and in the case of journal bearings ring bending adversely affects lubricant film generation and pressure distribution.